1. Field of the Invention
The present invention relates to a filter circuit and, more particularly, to a filter circuit using variable impedance devices.
2. Description of the Prior Art
In general, a filter circuit employs a time constant circuit for determining the filter characteristics, i.e., the frequency characteristics of the filter circuit. As is well known, time constant circuits are constituted by resistive devices and capacitive devices. The time constant circuit requires that the time constant be maintained stable at a predetermined value. When such a filter circuit is fabricated in an integrated circuit (referred as IC hereafter), it is necessary to adjust the time constant of the time constant circuit, since the resistances and the capacitances of the resistive devices and capacitive devices of the time constant circuits fabricated in ICs are not precise. Further, the resistances and the capacitances of the resistive devices and capacitive devices of the time constant circuits fabricated in ICs are unstable in relation to temperature change and long term use.
For the above reason, in ICs, variable impedance devices, such as MOS FETs (Metal Oxide Semiconductor Field Effect Transistor), are used for the resistive devices. This is because the variable impedance devices, such as MOS FETs, are favorable for adjusting the time constant of the time constant circuit.
Referring now to FIGS. 1 and 2, some conventional filter circuits using variable impedance devices, such as MOS FETs, will be explained. The filter circuits shown in FIGS. 1 and 2 are active and passive type second order filter circuits. Particularly, the active type filter circuit, as shown in FIG. 1, is a so-called Sallen-kye type filter circuit.
The Sallen-kye type filter circuit shown in FIG. 1 is comprised of two time constant circuits 10a and 10b and a buffer amplifier 12 which are connected in series between an input terminal 14 and an output terminal 16. Each of the two time constant circuits 10a and 10b has a variable impedance device comprised of a MOS FET 18a (18b), and a capacitor 20a (20b). The MOS FETs 18a and 18b of the time constant circuits 10a and 10b and the buffer amplifier 12 are connected in series between the input terminal 14 and the output terminal 16. The capacitor 20a of the preceding time constant circuit 10a is connected in parallel with the series circuit of the MOS FET 18b and the buffer amplifier 12. The capacitor 20b of the following time constant circuit 10b is connected between the drain of the second MOS FET 18b and a ground terminal 22. The gates of the MOS FETs 18a and 18b are coupled to a terminal of a source 24 for supplying a control signal Vc. Another terminal of the control signal source 24 is connected to the ground terminal 22.
An input signal Vin on the input terminal 14 is applied to the buffer ampliier 12 through the time constant circuits 10a and 10b in series, and an output signal Vout appears on the output terminal 16. Resistances R18a and R18b of the MOS FETs 18a and 18b vary in accordance with the control signal Vc fed to the gates of the MOS FETs 18a and 18b. Thus, the time constants T10a and T10b of the time constant circuits 10a and 10b can be adjusted to desirable amounts by the control signal Vc.
The transfer function of the active type Sallen-kye filter circuit, as shown in FIG. 1, is carried out as follows. If the resistances R18a and R18b of the MOS FETs 18a and 18b satisfy the relations R18a=R18b=R18, and the amplification factor K of the buffer amplifier 12 is 1, ##EQU1## wherein S is a constant given by the equations: S=1/(j.multidot..omega.) or S=1/(j.multidot.2.pi.f). In the equations, j is a unit imaginary number, .omega. is the angular frequency of the input signal Vin, and f is the frequency of the input signal Vin.
Hereupon, if the time constants TCa and TCb of the time constant circuits 10a and 10b satisfy the relationships: EQU TCa=R18.multidot.C20a, TCb=R18.multidot.C20b, (2)
The following equation is obtained: ##EQU2##
In case the filter circuit is fabricated in the IC configuration, the absolute values of the capacitances C20a and C20b are largely dispersed (to the extent of .+-.30%), but the relative accuracy is good. For example, if the capacitance C20a is enlarged by +10%. the capacitance C20b is enlarged also by +10%. Therefore, against this dispersion, if the gate voltage of the MOS FETs 18a and 18b, i.e. the control signal Vc is regulated, and the resistance R18 is reduced by 10%, the time constants TCa and TCb can be constantly maintained.
The only difference between the passive type second order filter circuit shown in FIG. 2 and the active type one, i.e., the Sallen-kye type filter circuit shown in FIG. 1, is that the former one lacks a buffer amplifier. Thus, the transfer function of the passive type filter circuit shown in FIG. 2 is almost the same as the transfer function for the Sallen-kye type filter circuit given by the above equation (3).
In the conventional filter circuits using MOS FETs, as shown in FIGS. 1 and 2, the time constants TCa and TCb of the time constant circuits 10a and 10b comprising the filter circuit are apt to vary in response to the input signal Vin applied to the filter circuit, as described later. As a result, the filter characteristics defined by the time constants TCa and TCb become unstable and worsen the S/N (signal to noise ratio) of the signal passing through the filter circuit.
That is, the input signal Vin applied to the filter circuit includes a DC component Vin(DC) and an AC component Vin(AC). Thus, the input signal Vin is given as follows, EQU Vin=Vin(DC)+Vin(AC)
The input signal Vin is applied to the source terminal of the MOS FET 18a. On the other hand, the control signal Vc is applied to the gate terminal thereof. Thus, the gate to source voltage VGSa of the MOS FET 18a is given as follows, EQU VGSa=Vin-Vc=Vin(DC)+Vin(AC)-Vc (4)
The equation (4) shows that the gate to source voltage VGSa of the MOS FET 18a varies in response to the AC component Vin(AC) of the input signal Vin.
The signal Vin passing through the preceding time constant circuit 10a is applied to the source terminal of the MOS FET 18b of the following time constant circuit 10b. On the other hand, the control signal Vc is applied to the gate terminal thereof. Thus, the gate to source voltage VGSb of the MOS FET 18b also varies in response to the input signal Vin, i.e., the AC component Vin(AC) of the input signal Vin.
The variations of the gate to source voltages VGSa and VGSb cause the impedances of the MOS FETs 18a and 18b to change so that the time constants TCa and TCb of the time constant circuits 10a and 10b vary. Accordingly, the filter characteristics of the filter circuit vary due to the variation of the input signal Vin, i.e., the AC component Vin(AC) of the input signal Vin. As a result, the S/N of the signal passing through the filter circuit is worsened by the variation of the input signal Vin. The deterioration of the S/N is particularly remarkable for frequencies around the cut-off frequency of the filter circuit.